This invention relates to magneto-optic recording media and, in particular, to magneto-optic recording media having a non-oxide dielectric layer of two different dielectric materials disposed between a transparent substrate and the recording layer.
The following methods are generally used to record information on optical recording media;
1. Condensed laser light is irradiated onto a magnetic recording layer in order to invert the magnetization of the magnetic recording layer; or
2. Laser light is irradiated onto a recording layer to change the crystalline structure of the recording layer, for example, from crystalline to amorphous, from amorphous to crystalline, from hexagonal to cubic, from cubic to hexagonal and the like, by utilizing the phase transition of the laser light irradiated onto the recording layer to transform the recording region by making holes, forming bubbles and the like.
A conventional magneto-optic recording medium is shown in FIG. 1. A plastic substrate 1 such as PMMA, PC, epoxy resin and the like having guide grooves is provided. A magneto-optic recording layer 2 such as terbium-iron (TbFe), gadolinium-terbium-iron (GdTbFe), gadolinium-terbium-iron-cobalt (GdTbFeCo) and terbium-iron-cobalt (TbFeCo) is formed on substrate 1. A dielectric film 3 such as silicon dioxide (SiO.sub.2) is formed on magneto-optic recording layer 2.
FIG. 2 shows another example of a conventional magneto-optic recording medium in which a glass substrate 4 is used in place of plastic substrate 1 of FIG. 1. In FIG. 2, glass substrate 4 is provided with an ultraviolet curing resin layer 5 for forming guide grooves on substrate 4. A magneto-optic recording layer 6 and dielectric layer 7 are formed on ultraviolet curing resin layer 5. However, both plastic substrate 1 of FIG. 1 and ultraviolet curing resin layer 5 of FIG. 2 have high water vapor absorption and high permeability to gasses. For this reason, magneto-optic recording layers 2 and 6 oxidize easily even with protective silicon dioxide (SiO.sub.2) dielectric film layers 3 and 7 provided thereon. Magneto-optic recording layers 2 and 6 can still oxidize on the side facing substrates 1 and 4. Such oxidation results in deterioration of magnetic properties, causes the film to peel off and creates other problems.
FIG. 3 shows an example of an improved conventional magneto-optic recording medium in which a second dielectric film 9 is formed between a substrate 8 and a magneto-optic recording layer 10. Magneto-optic recording layer 10 is sandwiched between two dielectric films 9 and 11. As shown in FIG. 3, substrate 8, which may be plastic as shown in FIG. 1 or glass with an ultraviolet curing resin layer as shown in FIG. 2, is provided. A dielectric film 9 such as SiO.sub.2 is formed on substrate 8, magneto-optic recording layer 10 is formed on dielectric film 9 and another dielectric film 11 is formed on the magneto-optic recording layer 10, as shown in FIGS. 1 and 2.
Another improved recording medium to which a reflective film 12 is added to the structure of FIG. 3 is shown in FIG. 4. The purpose of providing reflective film 12 is to enhance the magneto-optic effect of the medium rather than to improve weatherability. As shown in FIG. 4, reflective film 12 may be aluminum, copper or the like. Although the weatherability of the magneto-optic recording media is improved using this structure as compared with the structures of FIGS. 1 and 2, it is not completely satisfactory. This is because SiO.sub.2 dielectric films are oxides and free oxygen tends to oxidize the magneto-optic recording layer. In order to avoid this problem, it would be desirable to use non-oxide dielectric materials which do not yield free oxygen. However, only oxide films such as silicon dioxide (SiO.sub.2), titanium dioxide (TiO.sub.2) and aluminum trioxide (A1.sub.2 O.sub.3) adhere sufficiently to a plastic substrate such as PMMA, PC, epoxy resin and the like or to a glass substrate having an ultraviolet curing resin layer.
Non-oxide dielectric films such as silicon nitride, aluminum nitride and zinc sulfide have not been used because cracking occurs either during formation of the dielectric film or during weatherability testing. In order to make non-oxide dielectric materials that are suitable for use, methods for improving the weatherability have been introduced. For example, improvement in weatherability of magneto-optic recording layers has been attempted as described in Journal of Japan Applied Magnetics Society, Vol. 8, No. 2, 1984, p. 105, and in Summaries of Lectures in the 8th Japan Applied Magnetics Society Conference, p. 125. Alternatively, a protective film made of a metal, such as aluminum and titanium as described in Summaries of Lectures in the 8th Japan Applied Magnetics Society Conference, p. 148 is used. However, even using these improved methods, oxidation of the magneto-optic recording layer from the substrate side is not prevented.
Alternatively, the substrate itself and guide grooves provided thereon have been experimentally formed only of glass without an ultraviolet curing resin layer as described in Summaries of Lectures in the 7th Japan Applied Magnetics Society Conference, p. 155, and in Summaries of Lectures in the 8th Japan Applied Magnetics Society Conference, p. 239. While this improves the weatherability to a level that is good enough for use, formation of guide grooves on glass is not suitable for mass production because it is extremely expensive. For these reasons, suitable dielectric films for use on plastic substrates and on ultraviolet curing resin layers have not been found.
Accordingly, it is desirable to provide an improved magneto-optic recording media which overcomes the disadvantages found in prior art recording media.